There have been known electrically driven vehicles of a hybrid type in which a generator is driven by an engine and electric power generated by the generator is supplied to a motor for rear wheels to drive the rear wheels. When such an electrically driven vehicle of the hybrid type travels at high speed, the motor torque drops due to the increase in the revolution speed of the motor, and the braking distance of the vehicle can increase significantly especially when the vehicle carries a heavy load and travels on a downward slope or the like. When the braking distance increases, it is common to brake the vehicle by also using one of an exhaust brake device, a hydraulic brake device and a mechanical brake device in addition to an electric brake which uses the motor torque for the braking. When the braking distance increases and the braking time extends significantly, overheating of brake devices can occur especially in cooled brake devices such as the electric and mechanical brake devices and deterioration due to friction and heating can occur especially in the hydraulic and mechanical brake devices.
Therefore, in order to avoid the significant increase in the braking distance, there has already been proposed an automatic control unit that makes the vehicle speed equal to a preset control vehicle speed (see Patent Document 1, for example).